I once made a similar thing to help with transposing tunes. It consisted of two disks with a split pin connecting them at the centre. Both disks were marked with all twelve keys (you could arrange them as the circle of fifths if you wish). You simply lined up the keynote of original key the piece was in and the new key and read off the notes you should use. I found it very useful as I was always making mistakes when transposing and it helped to reduce errors.